Pumps for internal-combustion engine cooling systems have traditionally been belt-driven, at a fixed ratio, directly from the engine.
The coolant flow rate and pressure head required to effectively control the engine temperature are not, however, optimal when driven proportionally to the engine's rotational speed. The coolant system has to cope with the fully-laden vehicle struggling up-hill on a hot day, and the same system has to make sure the heater warms up rapidly in very cold conditions. Also, for efficiency, the energy consumed by the coolant pump ideally should at all times be only the minimum needed to just achieve the optimum temperature in the coolant. Whatever coolant circulation system is used, it must of course cater for the extremes; in the case of the traditional belt-driven coolant pumps, the need to cater for the extremes so compromises the efficiency of normal running that traditional coolant pumps are inherently non-optimal for most of their operating conditions.
The optimum coolant temperature is dictated by considerations of engine performance, fuel efficiency, exhaust emissions, etc. The coolant circulation system must provide a volumetric flow rate, and a pressure head, such that the coolant is cooled down (or warmed up) to the correct temperature under the extreme conditions. The invention is aimed at making it possible still to accommodate the extremes, and yet to improve the efficiency of the coolant circulation system during normal running, so that the system consumes only a minimum of energy during normal running.
When the coolant pump provides excessive flow and head, the engine wastes power and the overall engine efficiency is reduced.
When the coolant pump provides insufficient coolant flow and head, the engine runs too hot, thereby reducing engine performance, and perhaps damaging the engine.
Engine designers have not, in general, switched to driving coolant pumps by means of electric motors. This fact should be viewed in light of the fact that it is very common for a designer to specify that the engine's cooling fan to be driven by an electric motor. There, the motor runs at constant speed, and is controlled simply by being switched on/off: the need for switching is signalled by a simple electrical thermostat. That is a simple enough duty requirement for an electric motor to be subjected to.
It is recognized, however, that a simple on/off control would be far too crude for controlling the flow of coolant. Even under the minimum coolant flow conditions, the coolant must still be pumped and circulated quite vigorously.
It might be considered that, if an electrically-driven coolant pump were to be provided, it would be possible to control the coolant flow by controlling the rotational speed of the electric motor. Theoretically, this could be done by varying the electric current supplied to the motor that drives the coolant pump. However, such control of the motor speed by control of the motor current has not found favour with engine designers.
Thus, in considering the use of an electric motor to drive the coolant pump, it is apparent, first, that simple thermostatic on/off switching of a pump motor is out of the question, and second, trying to control motor-speed by controlling the current supplied to the electric motor has not found favour. And, even as a last resort, the notion of controlling coolant-flow by means of coupling the pump to a fixed speed motor by means of a mechanical variable speed drive, must be contra-indicated out as being far too elaborate; also, as mentioned, it is important that the pump, as well as the motor, should run at constant speed.
The invention is aimed at making it possible to vary the coolant flow to suit many different conditions, in a way which allows the pump (and hence the motor) to run at constant speed.